Intensified modulation of the Pacific north equatorial current bifurcation by the southern annular mode since the early 1990s

Long-term reanalysis data were used to assess inter-decadal to decadal modulations of the North Equatorial Current (NEC) bifurcation in the Pacific after the early 1990s. The wind stress curl anomaly (WSCA) in the region of 10° N–15° N and 160° E–170° E (C-BOX) had been found to excite Rossby waves and control NEC bifurcation along the Philippine coast. Our analysis revealed that the WSCA in the C-BOX has been remotely modulated by the Southern Annular Mode (SAM) since the early 1990s. It is shown that the SAM shifted to its positive phase at this transition and began strongly impacting the WSCA in the C-BOX and the NEC bifurcation. During the positive SAM phase after the early 1990s, strong climate variability occurred in the tropical to subtropical area of the North Pacific, with a clear footprint connected to the Antarctic region. Consistent with that finding, we determined that during the positive SAM phase, a dipole sea surface temperature pattern was generated in the South Pacific; this induced an atmospheric Rossby wave train in upper-level wind shear that propagated northward to the North Pacific. Such effects further enhanced downward motion and divergence at the surface, intensifying the easterlies in the equatorial area and the anticyclonic WSCA in the C-BOX. The anticyclonic WSCA in the C-BOX substantially excited downwelling oceanic Rossby waves at the surface, inducing an equatorward trend of NEC bifurcation after the early 1990s.


Scientific Reports
| (2022) 12:21210 | https://doi.org/10.1038/s41598-022-25661-w www.nature.com/scientificreports/ of Rossby waves and thereby impacts fluctuations in the NEC bifurcation. Their analysis demonstrated that the WSC in the C-BOX and the associated induction of Rossby waves are regulated jointly by the ENSO and Pacific Decadal Oscillation (PDO) 10,11 . They noted that Rossby waves are strongly enhanced when only one of the PDO and ENSO signals is in a strong positive phase 9 . Thus, the C-BOX appears to be deeply impacted by interannual to decadal climate variability over the Pacific region.
Considering that the C-BOX is situated over the North Pacific Subtropical Gyre (NPSG), variability in the NPSG is also strongly associated with variations in the WSC over the C-BOX region. Liu et al. 12 examined the effects of the Southern annular mode (SAM) on the South China Sea monsoon; they found that the SAM influences climate variability over the NPSG area. Furthermore, the remote modulation effect of the SAM on East Asian monsoons over the NPSG has been confirmed 13 . Since the monsoons might have an impact on wind field of the NPSG region where the C-BOX is located in, we inferred that the SAM might play an important role on the C-BOX variability and the consequential NECBL fluctuations. In addition to the local climate modulation associated with the ENSO and the PDO 9 , remote inter-decadal to decadal climate variability driven by the SAM might also play an essential role on WSC variations over the C-BOX area, with substantial impacts on the generation of Rossby waves and meridional displacement of the NECBL.

Results
Relationship among wind field, Rossby waves, and the NECBL. Previous studies 2,9 indicated that the cyclonic (anticyclonic) WSCA in the central Pacific may excite westward-propagating upwelling (downwelling) Rossby waves, which could be identified via lower (higher) sea level height. These processes eventually result in a northward (southward) displacement of the NECBL along the Philippine coast. As demonstrated in Fig. 1, the NECBL was shifted poleward in 1997 and from 2003 to 2004. After 2004, the NECBL was shifted equatorward (Fig. 1a, data based on Fig. 2a in the work by Qiu and Chen 3 ). Rossby wave activities in the area of the NEC fluctuation can be examined via the observed sea level height pattern averaged over 10° N-15° N. The sea level height exhibited a pattern that was consistently out of phase with the NECBL, which was negative during 1997 and 2003-2004 and remained positive after 2004 (Fig. 1b). This tendency shows that Rossby waves  (Fig. 1c). These features support the previous finding that cyclonic (anticyclonic) WSCA in the C-BOX generates westward-propagating upwelling (downwelling) Rossby waves, which subsequently induce poleward (equatorward) movement of the NEC bifurcation site. The correlation between the NECBL and the low-passed WSCA over C-BOX is 0.65, which is statistically significance at the 95% confidence level determined by a Student's t-test. Therefore, the WSCA in the C-BOX can be used to identify variations in the NECBL 5 . Moreover, decadal variability can be observed through this series of processes. Because of the limited availability of satellite measurements of oceanic data, we used the WSCA in the C-BOX to explore the decadal modulation effect of the SAM on variations in the NECBL.

Modulation effect of the SAM on wind variations in the C-BOX.
In addition to local climatic variability, the NPSG is influenced by polar climatic variability through the SAM 12,13 . Therefore, the wind field in the C-BOX, which is located in the NPSG region, is strongly influenced by the SAM. Figure 2a shows the relationship between the WSCA in the C-BOX and the SAM climate index during 1960-2019. The correlation between the WSCA and the SAM index over that entire period was − 0.29. Nevertheless, decadal variability exists in the relationship between the SAM and the WSCA in the C-BOX. The SAM index had a weak correlation with the WSCA before the 1990s, and then became nearly out of phase with WSCA after 1995. A 10-year running-correlation analysis was conducted to further explore this relationship (Fig. 2b). The running-correlation between the SAM and the WSCA in the C-BOX oscillated between − 0.2 and 0.2 before the 1990s. After 1995, the SAM became highly negatively correlated with the WSCA in the C-BOX, reaching − 0.6 in 1997. Our results imply that the SAM might start to have an influence on the north Pacific from the early 1990s as it shifted from the negative to positive phase (the red line in Fig. 2a), and gradually came to impact the wind variations over the C-BOX region after 1995.
To further explain the C-BOX-SAM relationship, Fig. 3 shows the wave spectrum of the WSCA in the C-BOX (Fig. 3a) and the SAM index (Fig. 3b). In wavelet analysis of the WSCA in the C-BOX, a high power was assigned to the decadal variability of approximately 128 months, which is inside the cone of influence and has strengthened since the 1990s. In the wave spectrum of the SAM index, a high power of the decadal variability around 128 months is also present before 1970 and after 1990. The wavelet coherence analysis (Fig. 3c) further

Mechanism
To clarify the decadal change in the Pacific climate after 1995, the oceanic and atmospheric structures in the Pacific were regressed onto the SAM index to contrast between two periods: 1960-1995 and 1996-2019 (Fig. 4). Compared with 1960-1995 (Fig. 4a,c), the period of 1996-2019 showed stronger regressions in both the oceanic and atmospheric fields (Fig. 4b,d). Because the SAM was positive following 1996, the regressions well represent the anomaly patterns. A cold sea surface temperature anomaly (SSTA) occurred in the southeastern Pacific and was extended to the central tropical region by the south-easterly wind anomalies via the wind-evaporation SST (WES) feedback. The entire temperate zone was occupied by a cold SSTA connected to the Antarctic area. This SSTA pattern reveals the footprint of the SAM, which is initiated in the polar area and expands northward to the entire Pacific region. More detailed analysis was conducted using the 850-hPa anomalous wind field and sea level pressure anomaly (SLPA). During the period of 1960-1995, the SLPA and atmospheric circulation were weak in the northern Pacific (Fig. 4c). In contrast, the period of 1996-2019 experienced an enhancement of SLPA and more apparent wind circulation pattern compared with 1960-1995, especially in the North Pacific. Strong easterlies occurred in the tropical area west of the dateline and enhanced the anticyclonic WSCA around the C-BOX (black rectangle), whereas strong westerlies were observed east of 150°W. These results demonstrate that active climate variability over the North Pacific after 1995 is strongly associated with modulation by the SAM. These regressed patterns confirmed that the SAM exerted a stronger control on the Pacific conditions from 1996-2019 than from 1960-1995. The impact of the SAM on C-BOX becomes much more stable since 1996 during its continuing positive phase. A recent study provided insights into the impacts of the SAM on the NPSG 12 . Based on those findings, Fig. 5 illustrates the sequence of oceanic and atmospheric effects induced by the SAM, and then links them with wind variations in the C-BOX. First, the positive SAM signal after 1995 favors a dipole SST pattern at the surface in the South Pacific. This dipole SST pattern can modify the subtropical jets around Australia, inducing alternating anticyclonic and cyclonic shear in the Pacific Ocean. Anomalous wave activities resulting from upper-level divergence/convergence anomalies can further excite the wave train, which propagates northward to the North Pacific (Liu et al. 12 , see also Fig. S1). Upper-level convergence anomalies around 0°N-40°N enhance descending motions, causing a high-pressure zone and divergence wind field at the surface; these findings explain the SLPA

Conclusion
Previous research demonstrated that the NECBL shifts northward in the presence of strong upwelling Rossby waves 1,2 . The WSCA in the C-BOX has been strongly associated with the generation of Rossby waves and impacts on fluctuations in the NEC bifurcation. In the current study, we found that the SAM, which had minimal impact on the NPSG during its negative phase before 1990, shifted into a dominant role on variations of the WSCA in the C-BOX region beginning in 1995. The influence of the SAM on the WSCA in the C-BOX was confirmed through wavelet analysis. A strong signal with a 128-month period appears in the power spectra of both the SAM and the C-BOX wind field, suggesting a strong modulation effect of positive SAM on the C-BOX since the early 1990s.
To further explore the decadal variability in the influence of the SAM on the NPSG and particularly the C-BOX, we regressed oceanic and atmospheric structures onto the SAM index as and contrasted between two periods: 1960-1995 and 1996-2019. Compared with 1960-1995, the regression patterns of 1996-2019 showed much greater variability in the distributions of SSTA, SLPA, and wind patterns, suggesting that the SAM exerted a stronger control on the Pacific conditions from 1996-2019 than from 1960-1995. Strong easterlies occurred in the tropical area west of the dateline and enhanced the anticyclonic WSCA in the C-BOX (black rectangle). Our results also indicate that the strong climate variability in the NPSG and the C-BOX after 1995 is closely associated with modulation by the SAM. Based on the findings in a previous study 12 , the pathway through which the SAM controls the WSCA in the C-BOX can be discerned. Because the SAM became positive in the early 1990s, a dipole SST pattern was generated in the South Pacific. The associated upper-level cyclonic and anticyclonic shear induced a Rossby wave train that propagated northward and eventually induced upper-level convergence at 0°N-40°N, enhancing downward transport and divergence at the surface. This divergence strengthened the mean surface easterlies over the western Pacific, thereby increasing the warm SSTA in the eastern Indian Ocean